The need for improved memory devices, memory media and memory processing for computers has been dramatically demonstrated by the increasing speed and computational power of modern computer with vastly more complex programs to access and store in memory.
The major factor which is determinant of the size and price of a computer is the memory.
The data storage requirements of new performance computers and multimedia applications of computers is very large, typically many giga-bytes (10.sup.12 bits) and even tera-bytes. New and improved, compact, low cost, very high capacity memory devices are required. These memory devices should be able to store many giga-bytes of information, and should randomly retrieve such information at very fast random access speeds demanded by practical applications of modern computing and data processing.
An optical memory offers the possibility of packing binary-stated information into a storage medium at very high density, each binary bit occupying a space only about one wavelength in diameter. When practical information are taken into account this leads to a total capacity of about 10.sup.11 bits for a reasonably-sized two-dimensional optical storage medium.
At the present two general classes of optical recording medium exist, namely phase recording medium and amplitude recording medium. The first is based on light-induced changes of the index of refraction (i.e., phase holograms) whereas the second refers to photo-induced changes in the absorption coefficient (i.e., hole burning) or two photon absorption processes.
Various optical memory devices have been proposed, including 3D optical memory described by Rentzepis in U.S. Pat. No. 5,268,862, and Rentzepis and Esener in U.S. Pat. No. 5,325,324, herein incorporated by reference, a three dimensional laser disc drive system described by Goldsmith et al. in U.S. Pat. No. 5,113,387, herein incorporated by reference, optical memory system and method described by Lindmayer in U.S. Pat. No. 4,864,536, herein incorporated by reference and optical system, based on photo-chromic substances described by Malkin in USA Patent Application U.S. Ser. No. 08/577707, herein incorporated by reference. Another three-dimensional optical storage memory system is described by Parthinopoulos et al. in Science, Vol. 245. Pages 843-845, August 1989, also incorporated herein by reference.
Generally, a photochromic material in all these devices changes color when irradiated with UV, visible or infrared radiation while in the ground state. The light is adsorbed by the ground state molecule, which then undergoes a photochemical reaction to form the photoinduced state. Preferable, the photoinduced state absorbs light at a different wavelength than the ground state of a molecule. The photoinduced state reverts to the ground state by thermal reversion or by being irradiated with light again, preferably light with a different wavelength than the light used to "read" the photoinduced state.
The photochromic material is incorporated within a 3D matrix that is transparent to the activating light. The material then is irradiated, preferable by a laser, at point within the matrix to photochemically change the light absorption of the photochromic material at a site within the matrix. The 3D memory device "reads" the information by irradiating the sites with light at a wavelength for which the photoinduced state has a high absorption and high fluorescence yield. The information associated with a location can be erased by irradiation thereof with light having wavelength causing reverse photochemical change the photoinduced state B back to the ground state A.
The problem of practical implementation of a concept of creating three-dimensional optical memory based on the above phenomenon of photo-chromism of organic compounds with fluorescent detection faces difficulty in choosing a suitable active media.
The active media should combine a multitude of physical-chemical properties simultaneously. Among these properties we can distinguish the most evident and simple ones
1) Molecules of the active media shall be adequate photochromic i.e. absorption bands of form A and form B shall have bigger extinction coefficients and shall not overlap. PA1 2) Molecules B shall possess photochemical and thermal stability. PA1 3) Quantum yields of direct (A to B, .psi..sub.B) and back (B to A, .psi..sub.A) photoreaction shall be sufficiently high and (.psi..sub.B &gt;&gt;.psi..sub.A. PA1 4) Form B of photochromic substance should fluoresce (fluorescence is the most sensitive spectral method allowing to detect minimum number of molecules). PA1 5) It is desirable that the quantum yield of this fluorescence should be as high as possible substantially exceeding the quantum yield of the reverse photo-reaction. Evidently it is a problem to combine simultaneously all desirable properties in one chemical material.
A variety of photo-chromic materials have been suggested for use as the active element in optical memory systems, J. Malkin et al. In Photochemistry of Molecular Systems for Optical 3D Storage Memory, Research on Chemical Intermediates, Vol. 19, No. 2, pages 159-189 (1993) (herein incorporated by references) suggests the use of spiropyrans as photochromic materials in 3D memory devices. Fisher et al. In U.S. Pat. Nos. 5,208,354; 5,177,2278 and 5,407,885, herein incorporated by references, describe the synthesis and the use of naphthacenequinones for the reversible optical storage of information.
The synthesis and use of naphthacenequinones as fluorescent dyes was described in U.S. Pat. No. 4,036,805 herein incorporated by references; the kinetics of photo-chromic effect were described by Malkin et al. In Photochromism and Kinetics of Naphthacenequinones, Journal of the American Chemical Society, Vol. 116, pages 1104-1109 (1994), herein incorporated by references. The synthesis and properties of naphthacenequinones were reviewed by N. T. Sokoluk et al. In an article, Naphthacenequinones: Synthesis and Properties, Russian Chemical Reviews, vol. 62, pages 1085-1024 (1993) herein incorporated by references.
The photo-chromic properties of anthraquinones have also been studied in the following articles by N. P. Gritsan et al.: Kinetic Study of the Photochromic Transformations of 1-alkyl-9, 10-anthraquinones, Russian Journal of Physical Chemistry, Vol. 64, 3081-3086 (1990), Experimental and Theoretical Studies of Photoenolization Mechanisms for 1-Methylanthraquinone, Journal of the American Chemical Society, Vol. 113, pages 9915-9920 (1991), and Mechanism of Photochromic Transformations of Peri-acyloxy-9,10- and 1,9-Anthraquinone Derivatives, Journal of Photochemistry and Photobiology, Vol. 52A, pages 137-151 (1990); all the above herein incorporated by references.
The application of diarylethene-type systems with heterocyclic rings was described in Jap. Patent 89,954. The synthesis and properties of diarylethenes was reviewed by M. Irie et al.: Thermally Irreversible Photochromic Systems, Journal of Organic Chemistry, Vol. 53, pages 6136-6138 (1988) and Journal of Organic Chemistry, Vol. 53, pages 803-808 (1988) and Design and Synthesis of Photochromic Memory Media in Photoreactive Materials for Ultrahigh Density Optical Memory; M. Irie (Ed.); Elsevier, 1994, pages 1-34.
The main problem associated with the use of the proposed spiropyrans as photo-chromic substrates in an optical memory devices is lies in the fact that their photo-induced form is not thermally stable and is capable to revert to the ground state by itself To prevent the undesirable spontaneous reversion to the ground state, the matrix carrying these photo-chromic materials must be cooled to at least -78.degree. C. and preferably lower temperatures. Necessity in such low temperatures as a precondition for efficient functioning of devices utilizing above materials is associated with difficulties in design and limits the scope of possible applications. Besides that spiropyrans usually lose their photochromic properties after a few read-write-erase cycles.
The main unsolved problem with the use all the known organic substances (including spiropyrans) in 3D memory devices based on fluorescence registration is the non-destructive reading. The readout stability of all known photo-chromic medium materials is not sufficient. This means, that even illumination with weak light used for reading can induce the erasing reaction, which is proportional to the numbers of photons absorbed by the media. Therefore after many readout cycles the memory is destroyed.